'77 T2 Haltech Injection T4 engine idle

[jonneeb]

Hi Brains trust.
I have a 1977 Dual cab Ute (Pick-up, I live in Australia)
Engine is really strong, drives great, starts every time, however, recently the engine idle is higher than it used to be.
On cold start it idles fine, then I drive it a short distance and the idle revs rise up to ~1500 RPM when I stop at a set of lights or a junction.
A quick blip of the throttle usually lowers the idle revs back to around 900-1000 as normal.
I've adjusted the idle by using the locknuts on top of the throttle body that clamp down on the cable return spring.
This has lowered the RPM slightly, but it still rises higher after driving.
When it's warm, the blip of the throttle will sometimes drop the RPM down to below 500 which pulls the oil pressure light on, a subsequent gentle push on the pedal will return RPM to 900-1000.
I've not touched the idle adjustment grub screw because it appears to be the return spring isn't returning to where it should be. Does anyone have an exploded drawing of the throttle body and spring assembly so I can see how it's supposed to work before I take it apart please?
Or, advice as to how it works and what the issue might be?
Cheers